Better API for misc

[martinpopel]

Currently, the misc attribute is treated as string, so we need to use code like $node->misc =~ /SpaceAfter=No/.
It would be more user friendly to access it (especially when editing) as $node->misc->{AttributeName}.
See related #4.

It is a question whether to deserialize misc always within Read::CoNLL-U or whether to do it only lazily when first accessed.
The latter option could be faster in cases no access is needed in the whole scenario (so there we can skip serialization from hash to string).